Plugins: Add synchronous CDN plugin loader (#99096)

* WIP

* Run plugin validations and validation steps sequentially if feature is off

* Remove dependency between sources.Service and pluginscdn.Service

* lint

* Parallelize validation only if class is CDN

* re-generate feature toggles

* remove waitgroup usage

* PR review: Add loader concurrency limit setting

* re-generate feature toggles

* pr review feedback

* fix const name

* Skip module.js validation for cdn plugins

* do not run validation steps in parallel

* lint

* reduce diff

* re-generate feature toggles

* lint

* pr review feedback

* remove leftover config.PluginManagementCfg from sources.Service

pkg/services/pluginsintegration/pluginconfig/config.go

@@ -30,9 +30,10 @@ func ProvidePluginManagementConfig(cfg *setting.Cfg, settingProvider setting.Pro
 		cfg.PluginsCDNURLTemplate,
 		cfg.AppURL,
 		config.Features{
-			ExternalCorePluginsEnabled: features.IsEnabledGlobally(featuremgmt.FlagExternalCorePlugins),
-			SkipHostEnvVarsEnabled:     features.IsEnabledGlobally(featuremgmt.FlagPluginsSkipHostEnvVars),
-			SriChecksEnabled:           features.IsEnabledGlobally(featuremgmt.FlagPluginsSriChecks),
+			ExternalCorePluginsEnabled:  features.IsEnabledGlobally(featuremgmt.FlagExternalCorePlugins),
+			SkipHostEnvVarsEnabled:      features.IsEnabledGlobally(featuremgmt.FlagPluginsSkipHostEnvVars),
+			SriChecksEnabled:            features.IsEnabledGlobally(featuremgmt.FlagPluginsSriChecks),
+			PluginsCDNSyncLoaderEnabled: features.IsEnabledGlobally(featuremgmt.FlagPluginsCDNSyncLoader),
 		},
 		cfg.AngularSupportEnabled,
 		cfg.GrafanaComAPIURL,